How to populate an array with numbers in JavaScript

Populate an array with numbers in JavaScript

The first step is to initialise the array. So today we are going to see a simple way to do it and see how we can populate an array with numbers in JavaScript. The idea is to populate it with the same number as many times as we want.

That is, if I want to have an array where the number 2 appears 10 times, I don't have to write something like this:

let myarray = [2,2,2,2,2,2,2,2,2,2];

The first thing to do is to create an array of the positions we want. To do this, we invoke the constructor of the Array class, indicating as a parameter the number of elements that the array will have:

let myarray = new Array(10);

In this case we will have an array prepared to contain 10 elements but it will be empty. That is why if we go through it we will see that there are no elements.

The next step is to fill the array with numbers. In this case we are going to fill it with the number 2 in its 10 positions. To do this we need to know the .fill() method of the Array object. The syntax of the .fill() method is the following:

arr.fill(value[, start = 0[, end = this.length]])

In its syntax we can see that the first parameter is the value with which we want to populate the array. The second parameter will be the initial position from where we want to populate it, this means that it is not necessary to fill the array completely, but we can do it partially. And the last parameter will be the final position where we will finish populating the array. This parameter is optional and in the case of not appearing it takes as value the size of the array, that is to say, it will populate up to the last position.

So, in order to populate our initial array with the 10 positions with the value 2 we will write the following:

myarray.fill(2,0,10);

However, we could have left out the last parameter and written the following:

myarray.fill(2,0);

Now, if we traverse the array to display its values, for example through a for each loop.

myarray.forEach(function(item,index,arr){      
console.log("Position " + index + ": " + item);  
});

We will see that the result per console is as follows:

Position 0 : 2  
Position 1 : 2  
Position 2 : 2  
Position 3 : 2  
Position 4 : 2  
Position 5 : 2  
Position 6 : 2  
Position 7 : 2  
Position 8 : 2  
Position 9 : 2

In this simple way and using the .fill() method we will have managed to populate an array with numbers in JavaScript.